About how many charge cycles has the pack had? You could drain from the harness (B18) with a small load. You are trying to determine two things; whether reducing the voltage of the one cell group will trigger charging (you are being the BMS) and whether or not the BMS has a balancing function or not after taking the “after” measurements. If one of the other groups gets close to 4.2v and charging stops and doesn’t resume, the BMS may not do balancing.thanks, I always charge those unknown batteries/projects outdoors, and even charge it inside an old bbq with some fans inside.
Any suggestion how can I drain just those high voltage cells? do I need to take the heat shrink from the battery and access each of the cells? or can I just drain from the B18 cable?
72V lithium battery only charge to 76V
- Thread starter sonoaatta
- Start date
eMark
100 kW
Is this battery several years old with hundreds of cycles? It may be nearing its End-Of-Life with lowest p-group at 3.6v while 4 oher p-groups are between 3.91v to 4.1v. Depending on age/cycles and more demanding usage it may be a loosing battle. having to manually balance after so many cycles when the charge voltage once again drops from say 82v to 78v resting voltage.. The battery may still suffice as a backup for less demanding duty ... assuming it says balanced from 82v to 80v for several cycles. .You may never be able to fully charge to 84v and a so-called dumb BMS just ain't good enuf and even a so-called smart BMS isn't either on a battery nearing its EOL.
To see if BMS is doing what it's suppose to do first discharge B18 and B19 to 3.7v. Then try charging again as red light should now indicate charge to at least 77.5v before going green. Next manually discharge B15 and B16 to 3.7v and see how long before red charge light changes before going green. Check total voltage again. See how many cycles you can get out of the battery before it only charges to 76v.
When you manually discharge you will get some bounce back voltage (depending on amount of amp discharge). So let it rest for 15-30 minutes and check again until resting voltage is 3.7v or better yet discharge to 3.6v figuring resting voltage will be at 3.7v.
With the price of a 1st rate quality battery we all want (to keep) our packs in good condition with hundreds of cycles as long as possible. This experience of not getting as many cycles as expected from a 20s battery tells all of us two things ...
(1). Was it a cheaper battery with no name Chinese cells of questionable quality?
(2). Did we overwork(abuse) the battery even having A-grade name brand cells?
You may have to periodically manually balance at least 4 p-groups having a so-called dumb BMS that uses resistance discharge (very slow) for so-so balancing of the 20 p-groups . With an older battery having endured hundreds of discharge cycles you will never be able to keep the parallel groups balanced like that of Harrisonpalm's well-balanced battery with an active equalizing BMS.
To see if BMS is doing what it's suppose to do first discharge B18 and B19 to 3.7v. Then try charging again as red light should now indicate charge to at least 77.5v before going green. Next manually discharge B15 and B16 to 3.7v and see how long before red charge light changes before going green. Check total voltage again. See how many cycles you can get out of the battery before it only charges to 76v.
When you manually discharge you will get some bounce back voltage (depending on amount of amp discharge). So let it rest for 15-30 minutes and check again until resting voltage is 3.7v or better yet discharge to 3.6v figuring resting voltage will be at 3.7v.
_________________________________
With the price of a 1st rate quality battery we all want (to keep) our packs in good condition with hundreds of cycles as long as possible. This experience of not getting as many cycles as expected from a 20s battery tells all of us two things ...
(1). Was it a cheaper battery with no name Chinese cells of questionable quality?
(2). Did we overwork(abuse) the battery even having A-grade name brand cells?
You may have to periodically manually balance at least 4 p-groups having a so-called dumb BMS that uses resistance discharge (very slow) for so-so balancing of the 20 p-groups . With an older battery having endured hundreds of discharge cycles you will never be able to keep the parallel groups balanced like that of Harrisonpalm's well-balanced battery with an active equalizing BMS.
Honestly I don't know anything about, I just found it in this used bike. Thank you for the suggestion I will try to drain B18 or each group try to get them to similar voltage then charge again. Thank you
It could be old, I just found it in this used bike, no details about it at all. It is a project I will just try to revive it for fun and learning, I have another battery to use on the bike. Thank you for the suggestion I will try to manually balance the cells from the harness then charge. I will report back.
Fun and learning is great, but please be careful. How are you planning on manually draining the cells via the balance wires?
Thank you, usually I have a small fan I use to drain slowly.
floydr
100 W
Bottom balancing might help, monitor the cell groups while discharging. B18 is the weakest cell Group it probably shot up to 4.2V and the BMS shut off. I suspect that B18 will drain faster than the rest of the cell groups.
The BMS wires being in sleeves points to a quality build.The BMS itself looks good from what I can see of it. Is there any printing on the BMS green PCBs?
I suspect the battery is nearing end of life that an active balancer won't correct
later floyd.
The BMS wires being in sleeves points to a quality build.The BMS itself looks good from what I can see of it. Is there any printing on the BMS green PCBs?
I suspect the battery is nearing end of life that an active balancer won't correct
later floyd.
eMark
100 kW
And sometimes it's a loosing battle, but still a valuable learning experience when time allows so you aren't rushed and frustrated
My thinking was being this is a learning project (no hurry up) that he would remove the wrap so he can discharge (not drain) the over-voltage from both B18 & B19 to 3.7 resting volts directly off both parallel groups at the same time ... as it's quicker and safer then the longer time required to safely discharge via tiny balance wires; while being extra careful not to accidently short and fry the tiny wires. Also discharge both B15 & B16 to 3.7V resting volts at the same time.
He may conclude it wasn't worth the effort if he has to continually rebalance the p-grops of an EOL battery. The battery may not even be suitable as a temporary experimental backup. Such a project tests one's patience when frustration and haste results in wasted effort. Still i'd try discharging B18-B19 to 7.4v and B15-B16 to 7.4v. Then try charging again to see if BMS will allow a charge voltage of 80v. Then put it back together and see how many charge-discharge cycles you can get to know whether it will suffice as a temporary backup battery holding at least 80v on a charge.
This is how I would do it as well, just to start to see if it's worth it. One thing to note, when I asked how he planned on draining cells:
It'd probably be okay, but specifics would be better. I'm assuming this is perhaps a simple computer fan? Not a bad option. It'd be better to measure the fan's resistance and use Ohm's law to find out how much current will be drained, because those balance wires aren't going to be able to carry much current.
Let's assume we want to drain just one parallel group at a time, say B18, and we'll want to drain at no more than 1 amp. V=IR, 4.1v/1 amp = we want to use a resistor with a value of no more than 4 ohms. If one were to use a higher value resistor, less current would flow, it would take longer. If we decide to drain 2 parallel groups at a time, and use the same 4 ohm resistor/load, 2 amps would flow (8v/4ohms = 2 amps)
Gruesome
1 kW
Has it been established that the OP's BMS is of the non-balancing kind?
Even if it is a balancing BMS, the deviation is massive, and it would be best practice to find out what caused it in the first place, before putting it into regular use.
Gruesome
1 kW
From my experience in unrelated but pretty technical areas, while it is always fun to take things apart, it's usually better to at least give the engineers who designed things a fair chance before saying 'eh, doesn't work', and ripping it apart. So why not let the BMS try to balance, even if it is very slow? I think the OP said he is charging outdoors.
eMark
100 kW
As has been stated in previous threads it could take "days" (if even then) to balance even a 10s battery with p-groups having deviation from 3.6v to 4.1v. Most passive resistance BMS balancing doesn't kick in unless the pack is close to fullly charged (84v) with the charger remaining on with green charging light remaining on for hours with p-groups that may only have an imbalance of say 0.150v to bring the imbalance to only 0.030v (30mV) ater several hours. So it makes some sense that AliExpress 18650 or 2170 less expensive batteries only use a BMS with overcharging, overdischarging, etc "protection" with a balancing function of little use.
Seldom if ever does anyone ever leave the charger on after the green light comes on for several more hours a couple times a month (or after every 10-12 charge/dischaarge cycles) for balancing the p-groups within 25-30mV of each other. Chinese ebike battery manufacturers came to the conclusion that only a "protective BMS" is really necessary with cells of questionable A-grade quality.
Thus the reason for the advent of the so-called Smart BMS to prolong useful life ALAP with an A-grade more expensive (quality) ebike battery. Getting as many useful cycles as possible with active equalized BMS balancing. Whether the expense of a Smart BMS is necessary for ebikers on a budget that can't even afford a quality A-grade battery and don't ever plan to build their own DIY battery is a moot-mute point
When this is not possible (second-hand used battery) it's probably best to get a new battery than getting frustrated trying to resurrect a 20s battery that is nearing or already at its EOL. FWIW when you say "drain" we understand it not so much as "bottom balancing" but rather as ... "siphon off the voltage from the highest charged group" ... with passive BMS balancing or manually balancing. When you say "drain" it reminds me and goatman and our 30Q experiments discharging the cells suffering from high self-discharge to 2.5V before recharging again with hopes of "regeneration" (no future signs of high self-discharge).
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